Of the numerous photocrosslinkable oligomers, acrylic epoxys and acrylic urethanes are now the most commonly used.
More recently, acrylic telomers obtained either by redox catalysis (as disclosed in French application FR-A-81 14 799) or by free radical initiating (as disclosed in French application FR-A-88 06 066) have been proposed. In any case, these oligomers are used in admixture with an acrylic reactive diluting monomer either in the presence of photoinitiators in the case of UV crosslinking, or directly, when electrons are used.
The setting times are very short, on the order of a second. This implies a high functionality in acrylic groups, which results in a great density of crosslinking nodes manifested by considerable shrinkages and stresses and a lack of adherence and flexibility, these phenomena probably being related.
Several solutions have been proposed, particularly the introduction of flexible sequences separating the acrylic groups. In the case of diacrylate silicones, the silicone sequence contributes the flexibility, and in the case of diacrylate polybutadienes, the butadiene sequence performs this role. Diol telecholic polybutadienes, with a flexible polybutadiene chain, are then involved, the grafting of the acrylics being performed on the diol ends.
Other proposed solutions consist of introducing groups promoting adherence to certain supports, such as acrylic grafted cellulose and acrylic phosphoric ester type adherence promoters.